Processes for making alpha alumina are known in the prior art. The following articles describe the principles of one such process that is closely related to the present invention: William M. Fish, "Alumina Calcination in the Fluid-Flash Calciner", Light Metals 1974, pages 673-682 and Edward W. Lussky, "Experience with Operation of the Alcoa Fluid Flash Calciner", Light Metals 1980, pages 69-79. The disclosures of such articles are incorporated herein by reference to the extent that they are not inconsistent with the present invention.
Other processes for making products comprising alpha alumina are described in the following patents: Newsome U.S. Pat. No. 2,642,337; Turpin U.S. Pat. No. 3,265,465; DuBellay et al U.S. Pat. No. 3,336,109; Hrishikesan U.S. Pat. No. 3,442,606; Reh U.S. Pat. No. 3,565,408 and Potter U.S. Pat. No. 4,224,288. However, each of these prior art processes suffers from one or more serious limitations making it less than entirely suitable for its intended purpose.
Currently, rotary kiln calcination is the principal method used commercially to produce alumina with a high alpha content. Rotary kiln calcination overcomes a stickiness and agglomeration effect that occurs in high temperature (above about 1220.degree. C.) fluidized beds of alumina. This stickiness effect, until now, has prevented the use of fluidized beds for the continuous production of aluminas with alpha content greater than about 65 wt%.
It is a principal object of the present invention to provide a process for transforming alumina hydrate into an anhydrous alumina product comprising alpha alumina wherein transformation to alpha alumina takes place in a reactor containing a bed that is fluidized with steam.
A related object of the invention is to provide a process for making alpha alumina in a fluidized bed at temperatures which may be greater than 1220.degree. C., while avoiding the stickiness effect that has heretofore prevented operation at such temperatures.
It is an advantage of the invention that the process will yield alpha alumina without addition of aluminum trifluoride or other mineralizing agent at an intermediate step. When no mineralizing agent is added, the alpha alumina product will not, therefore, be contaminated with residual fluoride or other mineralizing agent. Fluoride is known to be detrimental for high strength ceramic applications because it promotes crystal growth during sintering.
Another advantage of the invention is that it saves heat energy compared with prior art alumina calcination processes. Energy usage resulting from practice of the present invention is estimated at about 10-30% less than for rotary kiln calcination.
An additional advantage of the invention is that the alumina produced is more uniformly calcined than with rotary kiln calcination.
A further advantage of the invention is that the process does not require increased pressures and accordingly can be retrofitted to existing fluid flash calcination systems.
Additional objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification and drawings.